1. Field of the Invention
The present invention relates to a fuel assembly used in a nuclear reactor, and a thimble screw of the fuel assembly.
2. Description of the Related Art
An example of a nuclear reactor currently widely used for power generation includes a pressurized water reactor (to be referred to as “PWR” hereinafter). A fuel assembly used by the PWR is generally a canless fuel assembly with no wrapper tube. The structure of the canless fuel assembly will be briefly described. Top and bottom nozzles each having a plurality of coolant flow holes are connected to each other with a plurality of control rod guide tubes extending parallel to each other.
More specifically, the upper ends of the control rod guide tubes, i.e., so-called guide thimbles, are mechanically connected to the top nozzle, and the lower ends thereof are also mechanically connected to the bottom nozzle. These guide thimbles respectively accept the thin elongated control rods of a control rod cluster. Depending on the loading position of the fuel assembly in the core, the guide thimbles do not accept the control rods as they are not located at corresponding positions. In this case, the guide thimbles accept non fuel bearing components (NFBC) such as thimble plugs or burnable poisons. A plurality of grids are mounted on the guide thimbles. The fuel rods are accepted in the lattice openings and are elastically supported there.
Of the structure of the fuel assembly briefly described above, the structure of the connecting portion that connects the guide thimbles and bottom nozzle will be described in more detail with reference to the accompanying drawings.
FIG. 1 is an elevation schematically showing the structure of a fuel assembly applied to a PWR.
FIG. 2A is a sectional elevation showing part of the lower structure of this fuel assembly, and FIG. 2B is a bottom view of the same.
As shown in FIG. 1, a fuel assembly 1 has top and bottom nozzles 3 and 4 at upper and lower ends of elongated guide thimbles 2, and a top grid 5, middle grids 6, and bottom grid 7 fixed to the guide thimbles 2 in the longitudinal direction. Each of the top and bottom grids 5 and 7 is formed from a large number of lattice frames using thin plates, and holds fuel rods 8.
The top nozzle 3 is a bottomed box-like structure with a substantially square horizontal section. The top nozzle 3 has a plurality of coolant flow holes and guide thimble mounting holes in its end plate corresponding to the bottom plate. In addition, a hold down spring 9 is attached to the upper portion of the top nozzle 3. The bottom nozzle 4 has a top or end plate with a substantially square shape when seen from above, where a plurality of coolant flow holes and guide thimble mounting holes are formed. Legs 10 are respectively integrally formed to project from the four corners of the lower surface of the end plate.
The top and bottom nozzles 3 and 4 are connected to the upper and lower ends of the plurality of hollow tube-like guide thimbles 2 by utilizing the mounting holes described above.
Referring to FIGS. 2A and 2B, the lower end of each hollow tube-like guide thimble 2 is welded to a thimble end plug 12, and is fixed to the bottom nozzle 4 with a thimble screw 14 through an insert 13. One top grid 5 and seven middle grids 6 are mounted on the guide thimbles 2 at intervals, and the bottom grid 7 is mounted on the guide thimbles 2 through its connecting structure. It should be understood that the number of middle grids 6 can be appropriately changed.
The bottom grid 7 is fixed to the upper portion of the insert 13. A drain hole 15 extends through the thimble screw 14 in the axial direction, and a rotation preventive pin 17 for preventing loosening of the thimble screw 14 is provided to a seat 16 of the drain hole 15. The drain hole 15 allows the coolant in use to flow in the core in a direction P shown in FIG. 2A.
Furthermore, the seat 16 has a spot facing hole 18 communicating with the lower portion of the drain hole 15 and reaching the bottom surface of the seat 16. The rotation preventive pin 17 does not interfere with the flow of the coolant flowing into the drain hole 15 in the direction P.
The fuel rods 8 are inserted in and supported by the aligned lattice openings of the upper, middle, and bottom grids 5, 6, and 7 one by one, thus forming the fuel assembly 1.
With this structure, the drain holes 15 of the thimble screws 14 guide the coolant into the guide thimbles 2 in the core, and the introduced coolant cools the non fuel bearing components mounted in the guide thimbles 2. The drain holes 15 also serve as holes for sending the inner coolant to the outside.
During a scram mode of the nuclear reactor, the control rods are urgently inserted in the guide thimbles 2 by free fall. The drain holes 15 also serve as a restrictor for limiting the outlow velocity of the inner coolant so the fall impact is moderated. In other words, to assure the cooling function described above, the larger the diameter d of the drain hole 15 of the thimble screw 14, the better. To moderate the fall impact produced when the control rods fall, the smaller the diameter d, the better, which is contradictory.
During the scram mode of the nuclear reactor, when the control rods are urgently inserted in the guide thimbles 2 by free fall, an excessively large impact occurs to the top nozzle 3. For this reason, the guide thimbles 2 respectively have thin tube-like dashpots 20. The dashpots 20 reduce the velocity of the control rods falling in the guide thimbles 2, thereby moderating the excessively large impact acting on the top nozzle 3.
According to an example of the fuel assembly 1 with such dashpots 20, as shown in FIG. 3, a dashpot 20 with a length of 0.16 L to 0.18 L is provided to the guide thimble 2 where L is the length of the guide thimble 2 along its axial direction. Therefore, the compression load acting on the guide thimble 2 in the axial direction may cause flexural deformation of the dashpot 20. In this case, the control rod may not be inserted well.
For this reason, as shown in FIGS. 4 and 5, a technique is disclosed in which the length of the dashpot 20 of the guide thimble 2 is decreased. With this arrangement, the length of the dashpot 20 with respect to the length L of the guide thimble 2 can be suppressed to fall within the range of 0.03 L to 0.1 L, so the flexural rigidity of the dashpot 20 is increased. This can prevent flexural deformation of the dashpot 20.
This guide thimble will be referred to as an improved guide thimble hereinafter. The lower structure of a fuel assembly 1 to which an improved guide thimble shown in FIG. 5 is applied is different from that of the fuel assembly 1 shown in FIG. 2A only in that sleeves 21 are provided at the bottom grids 7 and that it has a dashpot 20 only at one portion, and is substantially the same as that of the fuel assembly shown in FIG. 2A.
In the fuel assembly to which the improved guide thimble is applied, the length of the dashpot 20 on the lower end side of the guide thimbles 2 is decreased, as shown in FIGS. 4 and 5. This increases the flexural rigidity of the dashpot 20 to prevent its flexural deformation. However, a so-called braking effect that moderates the fall velocity of the control rod is decreased.
In a PWR, its fall terminal velocity is limited from the viewpoint of ensuring the safety of the fuel assembly 1. Originally, the dashpot 20 is provided to the guide thimble 2 in an axial direction, as shown in FIG. 3, in order to moderate the fall velocity of the control rod such that the fall terminal velocity does not exceed a limit. For this reason, in the fuel assembly 1 employing the improved guide thimble as shown in FIGS. 4 and 5, a countermeasure that moderates the fall terminal velocity of the control rod must be provided by another means.